One of technologies used in optical communication systems to compensate deterioration of transmission characteristics along a communication path is transmission/reception equalization in the electrical domain or the optical domain. Various compensation technologies such as transmission equalization (pre-equalization) and reception equalization have been studied for the electrical domain. For the optical domain, too, various compensation technologies such as dispersion compensating fibers have been studied. Of those, the technology of transmission equalization in the electrical domain is a method by which favorable characteristics can be obtained without causing noise enhancement. In general, fiber installation cost and fiber installation location are issues in dispersion compensation that uses a dispersion compensating fiber in optical domain. However, employing pre-equalization has an advantage in that the cost and the number of places for installing dispersion compensating fibers are reduced. Pre-equalization transmission methods include a non-return to zero (NRZ) method and a return to zero (RZ) method. As compared with the NRZ method, the RZ method is characterized in that it is strong in intersymbol interference, and its average power is smaller than that of the NRZ method when the maximum amplitude of them is equal. However, the RZ method uses more bandwidth than the NRZ method. Therefore, it should be determined whether the RZ method or the NRZ method is applied according to use conditions.
Wavelength division multiplexing (WDM) transmission is known as a method that makes large-capacity optical communication possible. WDM transmission uses a plurality of wavelengths to transmit different signals that are multiplexed. Transmission power at which signals are transmitted to an optical fiber therefore fluctuates according to the number of wavelengths multiplexed, the method of modulation, and the like. An excess/shortage of the transmission power due to this transmission power fluctuation significantly affects transmission quality.
As a solution to the problem, a method of keeping the transmission power constant by inserting a variable optical attenuator (VOA) at the transmission end has been proposed (see Patent Literature 1, for example).
FIG. 6 is a schematic view thereof. A conventional optical communication system illustrated in FIG. 6 includes a transmission-side device a, a reception-side device b, and a transmission path c, which connects the devices. The transmission-side device a includes an optical amplification unit 1a, an optical filter 2a which lets an output of the optical amplification unit 1a pass, and a VOA 3a which is connected to the optical filter 2a as the last stage. The transmission-side device a transmits signal light while keeping transmission power at which the signal light is transmitted to the transmission path c constant by using the VOA 3a. The reception-side device b includes an optical filter 2b which lets signal light received from the transmission-side device a via the transmission path c pass, and an optical amplification unit 1b which is connected to the optical filter 2b. The reception-side device b lets the received signal light pass through the optical filter 2b and uses the optical amplification unit 1b to adjust reception power. The conventional optical communication system of FIG. 6 thus employs a method for keeping transmission power constant and realizing stable communication by providing the VOA in the transmission-side device.
Using pre-equalization transmission and WDM transmission in combination reduces the cost and the number of places for installing dispersion compensating fibers and makes large-capacity transmission possible as well. In pre-equalization transmission, the amount of dispersion compensation needs to be adjusted according to the amount of transmission path dispersion. Pre-equalization transmission power fluctuates greatly according to the amount of dispersion compensation. In the case where the amount of dispersion compensation is adjusted asynchronously with a pre-equalization signal multiplexed at a plurality of wavelengths, multiplexed WDM transmission signal light could cause an even larger instantaneous power fluctuation. Adjusting the amount of dispersion compensation therefore requires an optical amplifier large in gain and a VOA high in operation speed and wide in dynamic range that is capable of keeping transmission power constant in the wake of a large and instantaneous fluctuation in transmission power. This means that an expensive optical amplifier and VOA need to be mounted. Further, if the mounted VOA cannot keep up with transmission power fluctuations, there is a possibility of deterioration in transmission quality and a failure in the reception-side equipment.